1. Field of the Invention
The present invention relates to a modulator circuit for modulating a microwave oscillator having a negative resistance device comprising an IMPATT diode, and more particularly, to a microwave modulator circuit wherein the power conversion efficiency and reliability of the IMPATT diode is substantially improved.
2. Description of the Prior Art
Microwave frequency oscillators are increasingly using high frequency negative dynamic resistance diodes of the type now generally known as IMPATT diodes which is an acronym for Impact Avalanche and Transient Time. These diodes are increasingly being used as a source of fairly high power microwave energy such as at frequencies in excess of (1) Giga-Hertz (10.sup.9 cycles per second). An IMPATT diode is a semiconductor diode having a PN junction and a current transient region between opposite contacts. The IMPATT diode is generally mounted within a cavity resonator. When an applied direct current voltage exceeds the breakdown voltage of the diode, the diode is said to be in the avalanche breakdown region. Thus a slightly greater than breakdown voltage applied to the diode will cause a displacement current or electric field in the depletion layer of the diode's semiconductor material. Charge carriers are ionized at the point of maximum electric field within the depletion layer. The charge density is increased when the ionized carriers collide with other atoms and create more carriers forming a displacement current. The displacement current can also be considered as causing a wavefront, moving with a specific wave velocity provided that the magnitude of the current is sufficiently large. If the wavefront velocity associated with the displacement current is greater than the saturation velocity of the carriers, a high density of holes and electrons will be left in the wake of this wavefront. As a result of the concentration of holes and electrons, the electric field is then reduced and the velocity of the carriers is dimished leading to the formation of a dense plasma. Microwave energy is obtained from the avalanche diode by extraction of energy from the trapped plasma.
The RF voltages at the diode terminals give rise to the formation of concentrations of charge each of which travels across the transient region in the diode within the prescribed time period. The times for charge formation and transient through the diode are arranged with respect to the resonant frequency of an external resonator such that RF current that is 180.degree. out of phase with the RF voltage at the diode terminals flows through the resonator. Consequently, with an appropriate external resonator, this component of current through the terminals increases as the terminal voltage decreases, thus meeting the condition of negative resistance. Ultimately, part of the DC energy applied to the diode is converted to RF energy and the resonator and the circuit constitutes an reliable solid state microwave source.
Because of the solid state reliability of the IMPATT diode the use of such a diode is particularly desirable where lightweight and solid state reliability are imperative, such as in the target seeking radar system of a guided missile, or the like. In such portable armament, it is highly desirable that in addition to the improved reliability of the solid state device, that the radar system operate as efficiently as possible to reduce heat sinking needs and power drain from a power supply of limited capacity. It has been found after a study of the available microwave oscillators currently available that the commercially available modulators for the IMPATT diodes have efficiencies of only about 50-75% with rise times of only 20 or more nano-seconds and considerable on-state modulator losses. Accordingly, it is desirable to provide a reliable highly efficient solid state modulator circuit for a microwave oscillator having greatly improved efficiency.
Two of the reasons for the poor efficiency of the prior art modulators are poor transient time of such modulators since the modulator will dissipate unnecessary power during the transient time of the modulating waveform and detrimental on-state power losses. For instance, at zero voltage across the modulator and a maximum current through the modulator, the power dissipation will be minimal, and at maximum voltage across the modulator with minimum current through the modulator, the power of dissipation will likewise be minimal. During the transient time between minimum voltage and maximum current switched state, and the maximum voltage and minimum current switched state, substantial voltage and current will simultaneously appear respectively across and through the modulator causing considerable power dissipation in the modulator. Accordingly, it is desirable to switch the IMPATT diode between the two switched states with minimum transient time and co-existing minimum modulator on-state power dissipation for providing highest efficiency.